1. Field of the Invention
The present invention relates to a photoelectric input apparatus for display systems, and more particularly to an electric control system for a photoelectric touch input apparatus adapted to be placed in front of the display surface of a display device such as a cathode ray tube, a liquid crystal display panel or the like.
2. Description of the Prior Art
In U.S. Pat. No. 4,243,879 issued on Jan. 6, 1981, there was proposed a photoelectric touch input apparatus which comprises a frame adapted to be placed in front of the display surface of a display device and defining an opening therein, a plurality of light-emitting elements mounted on the frame in a row along one side of the opening to produce a plurality of light beams in parallel spaced relationship on a common plane, and a plurality of photosensitive elements mounted on the frame in a row along the opposite side of the opening and aligned with the light-emitting elements to receive the light beams. In such a conventional photoelectric touch input apparatus, one of the light-receiving signals of the photosensitive elements drops in level when one of the light beams related thereto is interrupted by an operator's finger. As a result, a switchover signal is produced to switch over an indication or picture on the display surface to another indication or picture defined by the position of the operator's finger.
During use of this touch input apparatus, however, it has been found that there occur relative differences in quantity of light between the light-emitting elements due to relative differences in light-emitting characteristics of the light-emitting elements. Such a phenomenon is caused by change of the ambient temperature, use for a long period of time and the like. Similarly, relative differences between levels of the light-receiving signals are caused by relative differences in light-receiving characteristics of the photosensitive elements. For these reasons, it has been required to carefully select light-emitting elements and photosensitive elements having the same characteristics during a mounting process thereof. Furthermore, it has been required to position the light-emitting elements and photosensitive elements in place on the frame without causing any relative displacement.